Augmented reality (AR) viewing may be defined as a live view of a real-world environment whose elements are supplemented (e.g., augmented) by computer-generated sensory input such as sound, video, graphics, or GPS data. Virtual reality (VR) viewing may be defined as a fully simulated world, within which the viewer may interact. A head-mounted display (HMD), also sometimes referred to as a helmet-mounted display, is a device worn on the head or as part of a helmet that is able to project images in front of one or both eyes. An HMD may be used for various applications including AR or VR simulations. HMDs are used in a variety of fields such as military, gaming, sporting, engineering, and training